The invention relates to a switching-valve arrangement for a shifting system of an automatic transmission, in particular for a shifting system of a dual clutch transmission, which has a first actuator and at least a second actuator for engaging or releasing gears.
DE 101 34 115 discloses a hydraulic control arrangement for controlling a dual clutch transmission which has a plurality of gears and a plurality of actuators for engaging and releasing the gears. The hydraulic control arrangement can be divided up here into a first and second partial circuit.
A first switching valve is provided between the first partial circuit and a pressure line, the switching valve, in a first position, separating the first partial circuit from the pressure line and, in a second position, connecting it to the pressure line. By virtue of the first switching valve being subjected to a first switching force by a valve-activating means, the first switching valve is moved from the first position into the second position counter to the force of a spring. It is thus possible to connect a first actuator within the first partial circuit to the pressure line via the first switching valve. If the switching force is reduced or set to zero, the spring forces the switching valve back into its first position.
Furthermore, a second switching valve is provided between the second partial circuit and the pressure line, this second switching valve, like the first switching valve, in a first position, setting the second partial circuit to a pressureless state and, in a second position, connecting the second partial circuit to the pressure line. Here too, a valve-activating means provides a switching force, a second switching force, in order to move the switching valve into the second position counter to the force of a spring. A second actuator of the dual clutch transmission, which in this case is to be assigned to the second partial transmission, can thus be connected to the pressure line.
Consequently, the two partial circuits can be set to a pressureless state and/or subjected to pressure by the first and the second switching valves. The two switching valves are switched by valve-activating means which are independent of one another. In the event of a valve-activating means for one switching valve failing, this independent activation may result in the two actuators being subjected to pressure simultaneously. In respect, for example, of the switching valves of DE 101 34 115, which are designed as electrically activated magnetic switching valves, this would be the case when, as a result of error, one of the two switching valves is permanently activated and the other switching valve switches as envisaged. However, irrespective of the way in which the hydraulic control arrangement according to DE 101 34 115 is actually divided up, error-induced simultaneous pressure activation of actuators of a shifting system can, in the case of dual clutch transmissions, result in serious damage. If, for example, the first actuator and the second actuator are assigned to a partial transmission of a dual clutch transmission, and if these two actuators are subjected to pressure simultaneously, it would be possible for two gears to be engaged simultaneously in this partial transmission.
It would indeed be possible for one switch-over valve, instead of the two switching valves, to be switched upstream of the actuators, this switch-over valve, in a first position, connecting the first actuator to the pressure line and, in a second position, connecting the second actuator to the pressure line. This rules out the situation where the two actuators are subjected to pressure simultaneously. On the other hand, if use is made of this switch-over valve, it would always be the case that one actuator is subjected to pressure, and this would be associated with certain leakage-related losses. It is indeed possible to provide upstream of the switch-over valve, in turn, a switching valve which, as has already been described above, separates the downstream actuators from the pressure line as required. If, however, this switching valve fails, for example if it cannot any longer be moved into the position in which it connects the pressure line to the actuators, it is no longer possible for the downstream actuators to operate any gears. This would possibly mean, in the case of a dual clutch transmission in a motor vehicle, that it is no longer possible to engage starting gears.